Study of DC-bias current distribution in AC power system based on direct field-circuit coupled model

DC transmission system in monopole-grounded return operation mode may lead to DC magnetic bias of some transformers in AC power system. In order to develop the management measures of DC bias effectively, it is necessary to study the distribution of DC magnetic bias current in AC power system. Based on the direct field-circuit coupled method, considering the effect of multi-layer soil model on the electrical current field and the impact of the transmission lines on the earth surface potential (ESP) distribution, the DC magnetic bias calculation models of ±800kV Xi-Zhe and ±800kV Ling-Shao UHV transmission project were built. The modeling range and the ground grid parameters were determined, while the soil model is modified as well. The calculated values of DC biasing current were compared with the test result and the total average error of each testing station is no more than 5%, which verifies the accuracy of the calculation model. This research can provide reference to the calculation of DC current distribution in large AC power system and the management of DC magnetic bias.